Several patients with inhibitors of blood coagulation have been identified, and in three, their inhibitors have been characterized in preliminary studies. One appears to have a heparin-like inhibitor. The other two are an IgM and an IgG lupus-like inhibitor. The heparin-like inhibitor has been purified by affinity chromatography (Agarose bound anti-human IgG and IgM) with further studies pending.